Mist oil lubrication process

ABSTRACT

Undesirable stray mist in mist oil lubrication is reduced to low levels by means of a lubrication process utilizing a mist oil composition containing as a stray mist suppressant from 0.001 to 2 percent by weight of certain oil-soluble polyolefins of viscosity average molecular weight greater than 5,000.

United States Patent [191 Altgelt et al.

[ Apr. 23, 1974 MIST OIL LUBRICATION PROCESS Inventors: Klaus H. Altgelt, San Rafael; Carl C. Thut, Richmond, both of Calif.

Assignee: Chevron Research Company, San

Francisco, Calif.

Filed: July 19, 1972 Appl. No.: 273,254

US. Cl 184/1 E, 252/59, 252/15,

252/305 Int. Cl. C10m 1/18 Field of Search 252/15, 59, 305; 184/1 E References Cited UNITED STATES PATENTS 11/1963 Gordon et al 252/59 X 8/1966 Anderson 12/1970 .lacobsson et al 252/59 X 3,389,087 6/1968 Kresge et a]. 252/59 3,676,521 7/1972 Stearns et a1.

3,510,425 5/1970 Wilson 252/305 FOREIGN PATENTS OR APPLICATIONS 1,099,450 1/1968 Great Britain 184/1 E Primary ExaminerCannon W. Attorney, Agent, or Firm-G. F. Magdeburger; C. J. Tonkin; S. R. La Paglia 5 7] ABSTRACT 6 Claims, 2 Drawing Figures AMOUNT OF MIST GENERATED PER MINUTE IN EACH DROP SIZE,mg/,U

PATENIEI] APR 2 3 I974 k-STRAY MIST SUPPRESSION OF STRAY MIST BY POLYMERS x PAMA a: 1.0- k- U) 0 PAMA o l I I I l 70 POLYMER IN o|| Fl (3.1

DROP SIZE DISTRIBUTION OF OIL MIST DROP DIAMETER,,U

FIG.2

MIST OIL LUBRICATION PROCESS BACKGROUND OF THE INVENTION 1. Field of the Invention A mist oil lubrication system pneumatically distributes fine droplets of an oil composition to the areas of various machine elements to be lubricated. The present invention is related to improvements in mist oil lubricating processes.

Mist oil systems have proven their reliability and usefulness in a variety of applications. They are used in various industries for services ranging from light duty service, e.g., dental equipment, to very heavy duty services, e.g., lubrication of steel mill backup rolls. Mist lubrication can provide the benefits of clean lubrication with little outside contamination, as well as effective and uniform lubrication, centralized distribution and low lubricant consumption.

Mist lubrication works by generating an oil mist, micro-fog, or aerosol which is transported pneumatically by compressed air, or other gas, to the area to be lubricated. Here, the aerosol is condensed, coalesced, or in the terminology of mist lubrication reclassified, by impact against a surface at high velocity.

The oil mist can be generated in various ways. A preferred system uses a device consisting of a reservoir opened to a venturi. Compressed gas is blown through the venturi, lubricant is drawn from the reservoir by the suction thus created and the lubricant is mechanically fractured by the turbulence of the air stream into tiny droplets. Downstream, the mixture impinges against baffles where large droplets that are transported with difficulty are coalesced and returned to the reservoir. The remaining oil particles form an aerosol with particle diameters in the range from 0.1 to 20 microns. Relatively large amounts of air, or gas, perform the pneumatic transport of the lubricant. The concentration of oil in the delivered mixture is of the order of 0.004 pounds per pound of air.

Reclassification is usually accomplished by valves or orifices (reclassifiers) which serve to direct and accelerate the aerosol so that it ultimately wets the surface to be lubricated, or the reclassifiers condense the aerosol and drip lubricant. Wetting of the surface occurs on impingement because particles in smoke, mist or aerosols can be coalesced by impact against a surface at high velocity. Just as in the use of a can of aerosol spray, if the spray nozzle is close to a surface, the high velocity aerosol entirely condenses in a small area, but if the spray nozzle is further away, air frictional losses reduce the aerosol velocity so that only a fraction of the liquid is collected at the surface and the remainder is lost as stray mist." The choice of the proper orifice and aerosol velocity for lubrication depends on the type of element to be lubricated and a number of factors related to the condition and composition of the mist lubricant, such as, oil viscosity, aerosol particle size and concentration, additives present, temperature, molecular weight, and chemical constitution.

In general, smaller particles require higher velocity to reclassify the oil while large aerosol particles, as in a visible mist, i.e., fog, readily wet surfaces at low velocity. Particles of 6 microns diameter begin to coalesce at even 25 feet per second (0.3 mph) and have good wetting at 70 feet per minute, while a micron particle must impact at 5,000 feet per minute (57 mph) for reclassification. Such velocities are normally unobtainable in practice with the consequence that the smaller diameter droplets form a stray mist.

At points in the mist oil system where lubrication is desired, the velocity of the aerosol must be great enough to provide wetting, yet the aerosol must be pneumatically transported to that area sufficiently slowly to avoid excessive wetting and lubricant loss in the pipelines. if large oil droplets are present in the aerosol, the droplets will readily wet and lubricate a bearing or other surface, but also have a tendency to condense within the feeder pipelines. lf particle size is too small and velocity too low at the element to be lubricated, coalescence will not occur and stray mist will present a serious problem. A very fine aerosol is difficult to coalesce by reclassification and excessive stray mist is produced giving a smoky effect to the atmosphere. Of course, the mist is generated with a distribution of particle sizes, so that stray mist, which arises from the low end of the mist droplet size distribution, i.e., droplets with diameters usually less than 0.5 microns, is a difficult problem to eliminate.

While great attention will be given to the misting and condensing ability of the oil, there are other properties required of a mist oil lubricant, such as, viscosity, lubricity, anti-wear, extreme pressure protection, oxidation stability, freedom fromdeposit formation, and corrosion and rust protection.

Stray mist is the most troublesome feature of mist lubrication systems. The lubricated machine element is normally open to the atmosphere and mist which is not reclassified escapes into the atmosphere where it may form a potential hazard to health and safety due to deposition on environmental surfaces and respiration. Mechanical precipitating schemes have been devised to collect stray mist, e.g., centrifugal precipitators. These devices are capable of removing particles down to 0.2 microns but they require auxiliary ducting and are not widely used. They are often prohibitively expensive.

2. Prior Art Additives to the mist oil have been far from successful in achieving the necessary reduction in stray mist without degradation of the other needed properties of mist oil lubricants. Polymers of the type which are normally used as pour point depressants (polyesters) or as viscosity index improvers in amounts sufficient to .be operative for that purpose, overly increase the viscosity of the oil, decrease its shear stability, or render it undeliverable pneumatically, or finally, fail to reduce the percentage of stray mist to below a few percent at reasonable weight percentages of additive. Increasing concern about pollution, operating plant cleanliness and the potential health and fire hazards posed by stray mist, has led us to focus our attention on this problem. The polymers of the present invention, while serving to drastically reduce stray mist, do not degrade the other performance characteristics of the mist oil lubricant.

Polymers are regularly used in lubricating oils in appreciable amounts (3-40 percent by weight) as pour point depressants, viscosity index (VI) improvers and dispersants. Polyesters (e.g., polyalkylmethacrylate) have been used as stray mist suppressants, U.S. Pat. No. 3,510,425, although they are not as effective for this purpose as polyolefins. British Pat. No. 1,099,450 teaches the use of certain polyester and polyisobutene polymers which are used as VI improvers and have low shear stability, as stray mist suppressants. Additives of the prior art were directed toward the reduction of percentage stray mist to about 2-10 percent. A mist oil lubrication process is regarded as commercially satisfactory if operated with below about percent stray mist.

SUMMARY It is found that undesirable stray mist in mist oil lubrication is reduced to surprisingly low levels (less than 1 percent) by means of a lubrication process in which the mist is pneumatically transported to the area to be lubricated and there reclassified without significant stray mist and without loss of the needed properties of a mist oil lubricant when the lubricant composition contains as a stray mist suppressant from 0.001 to 2 weight percent of certain oil-soluble polyolefins of viscosity average molecular weight greater than 5,000. The polyolefins are preferably C C copolymers having non-bulky backbone chains and a minimum number of side chains of minimum length to maintain oil solubility. x is equal to from 3 to about 12 with the proviso that one of the olefinic components of the copolymer must be ethylene. Especially preferred are (ethylene-propylene) C C copolymers of 40-80 mol percent ethylene and viscosity average molecular weight about or greater than 20,000. As a direct consequence of the use of the mist suppressants of the present invention it is found that stray mist is reduced to levels much lower than those achieved heretofor. Furthermore, the reduction in stray mist is accomplished with lower percentages of polymer. These polymers do not degrade the other performance characteristic of the mist oil lubricant.

BRIEF DESCRIITION OF THE DRAWINGS FIG. 1 shows the suppression of stray mist by various polymers. The open circles show the rapid suppression of stray mist to very low levels (less than 0.3 percent stray mist) by very small quantities of the preferred C C copolymer of molecular weight 100,000. The triangular points show the effectiveness of a polybutene (PB) of molecular weight 50,000. The scattered two points (PAMA) correspond to polyalkylmethacrylates of 30,000 and 500,000 molecular weight respectively, the more effective methacrylate (considering the lower concentration) being that of higher molecular weight.

FIG. 2 shows the number of milligrams of mist per micron of droplet diameter produced each minute from the nozzle of a misting manifold according to the test method herein described. This is the accepted method of plotting the drop size distribution of the oil mist in order to emphasize the amount of oil found in the straymist drop size region (below 0.5 micron droplet diameter). Note that the amount of oil in the stray mist range of droplet sizes is severely reduced by the addition of polymer. The solid circles refer to the base oil, a 500 SSU at 100F neutral petroleum oil with no polymer present. The triangular points refer to a composition of the same base oil containing 0.06 percent by weight of a polymethacrylate polymer of 30,000 molecular weight. The open circles refer to 0.05 percent by weight of a C C copolymer of 150,000 molecular weight in the same base oil. FIG. 2 graphically illustrates the stray mist suppressant effectiveness of the copolymers of the present invention. There is a vanishingly small amount of stray mist produced by the base oil containing only 0.05 weight percent of C C copolymer, while the base oil alone produces substantial stray mist, and the polymethacrylate-containing composition still has the initial sharp rise in amount of stray mist at low droplet size.

DETAILED DESCRIPTION OF THE INVENTION Surprisingly, low levels of stray mist (less than 1 percent) are produced by a method of mist oil lubrication which utilizes oil compositions containing very small percentages (0.001-2 percent by weight) of certain specified polyolefins. These polyolefins are found to (l) have a non-bulky polymer backbone, (2) have a minimum number of side chains and side chains of the minimum length to maintain oil solubility, (3) have viscosity average molecular weights in excess of 5,000 and preferably greater than about 20,000, obtained either by polymerization of monomers or degradation of polymers of higher molecular weight, and (4) shift the particle size distribution in the oil mist toward larger droplet sizes.

These qualities are found in oil-soluble C C copolymers of ethylene of viscosity average molecular weight greater than 5,000 and preferably about or greater than 20,000, which are 40-80 molar percent ethylene, wherein C is a C C mono-olefin, with the proviso that the polymers be oil-soluble. The term copolymer includes polymers derived from two or more dissimilar monomers. Of these, the C C copolymers are preferred.

The success of the mist oil system depends on a change of physical state of the lubricant from a bulk liquid to an aerosol and back again to bulk liquid. These two mechanisms, mist generation and reclassification, are influenced by a variety of oil properties. Some oils are very easily misted; the lighter, lower viscosity oils being especially so. From the standpoint of maximum efficiency of misting, transportation, and minimum recycling, lubricants that produce more particles in the system of about l-l0 microns in size are most desirable.

Reclassification, on the other hand, depends upon the size of the particles and their velocity. Inefficient reclassification results in stray mist that is unusable as lubricant.

The polymers of the present invention function to reduce stray mist by the suppression or elimination of the smallest droplets in the aerosol, those of diameter below 0.5 micron. The details of the mechanism by which the addition of polymers affect an oils propensity to form aerosols and to be reclassified have not been entirely defined. Viscoelastic properties, electrostatic charge density and drop size combined with aerosol concentration are factors. While the preferred polymers (C C copolymers) have been shown to be reasonably shear stable, it is not believed that shear stability is a critical factor in the choice of polymers for their ability to suppress stray mist. Specifically, a 100,000 molecular weight C C copolymer composition in 500 SSU at F neutral oil showed only 0.3 percent stray mist when said copolymer was present in the oil to the extent of 0.05 percent by weight. The same copolymer showed only ll percent viscosity index loss in a shear stability test.

It is known from the work of J. W. Gibbs and others on the surface free energy, or surface tension, that the thermodynamic properties of the bulk liquid differ substantially from those of the surface or interface. Similar differences are expected and have been found for viscoelastic properties. For drops of moderate size, the ratio of bulk liquid to liquid at the surface is quite large and the properties of the bulk liquid predominate. However, for the smaller droplets which make up mist lubricants, this is no longer true. Thermodynamic, and viscoelastic, surface properties become more important in mists than those of the bulk for many purposes, including particle size distribution, solubility, adsorption, etc. This makes it difficult, if not impossible, to draw conclusions about these properties of the mist from a knowledge of the bulk liquid.

It has been shown herein that polymers have an effect on the aerosol particle size, and most importantly, on the size distribution of the droplets. The proper selection of polymers provides an aerosol with a size distribution shifted to larger droplets. It is believed that by varying the polymer type and concentration, the amount of difficulty in coalescing particles can be minimized.

LUBRICANTS With the use of the higher viscosity lubricants as are required, for example, in steel mill bearings, it was at first believed that there would be little need for stray mist suppressants. It was believed that the high viscosity of the lubricant would not permit the formation of small droplets (below 0.5 microns) that form the stray mist. This premise is now believed to be faulty.

The lubricating oil base of the compositions of this invention can be a mineral oil or a synthetic hydrocarbon oil of lubricating viscosity, i.e., with a viscosity in the range of 35 to 50,000 SUS at 100F. While the oil may be paraffinic, naphthenic, or mixed base, it is preferred that it be paraffinic for oxidation stability. The characterization of an oil as paraffinic, naphthenic or mixed base is based on a number of factors described in Nelson, Petroleum Refining Engineering (4th Ed., McGraw-Hill Book Co., Inc., 1958). The oil may be a single refinery cut, or may be a blend of two or more oils in appropriate proportions to give the desired viscosity of the blend. A typical example would be an approximately 40-60 blend of a neutral oil having a viscosity of about 500 SUS at 100F. and a bright stock having a viscosity of about 4,000 SUS at 100F. The lubricating oil will represent the major portion of the composition of this invention and will comprise about 90-98 percent by weight of the composition, and preferably, about 93-96 percent by weight. Viscosity is usually dictated by the requirements of the machine element to be lubricated. Mistability is adjusted by control of viscosity in the mist oil system through heating of the oil or air at the mist generator. Mist generators without heaters are generally limited to oils with viscosities less than 1,000 SUS at the minimum operating temperature. From the standpoint of the mist system, the lowest viscosity oil that will do the lubrication job is the most desirable. Oils that are very high in wax content may congeal and block the very small openings of the reclassifiers, i.e., oils that have cloud points at or above room termperature. Obviously, oils with this limitation are not satisfactory.

Synthetic lubricants such as the alkyl, aryl, and alkaryl phosphate esters, alkyl benzenes, polyoxyalkylene esters or glycols, ortho silicates, siloxanes, etc., are also useful base oils for the mist oil compositions of the present invention when used separately or in miscible combinations of lubricating viscosity.

In mist oil processes with oil and/or air heaters, lubricant stability characteristics are of critical importance. As the oil is often subjected to continuous recycling, it must be thermally stable and resistant to oxidation. To this end, oxidation inhibitors are added to the composition. Several other types of additives are also commonly employed in these industrial oils to achieve performance. These additives function as rust inhibitors, oilincss agents, extreme pressure agents, anti-wear additives, detergents, foam inhibitors, metal deactivators, metal wetting agents, etc. These additives are well known in the prior art, a typical anti-oxidant, for example, is zinc dialkyldithiophosphate. Pour point depressant polymers, such as the polyalkyl methacrylates, can also be present. The totality of these other additives constitutes from about 1 to about 10 percent by weight of the composition.

The composition may be prepared simply by blending together the various components. Typically all minor portion components will be added to the base oil; they may be added neat, or as concentrates in oil solutions, where the solvent oil is mistable and compatible with the base oil. The components may all be blended simultaneously or, if desired, two or more of the components may be blended separately and the mixture then further blended with the remaining components to form the final composition.

TEST METHOD Percent stray mist was measured directly using an Alemite mist generator operating at 20-30 inches of water manifold pressure and at 60200F. air and oil temperature. The aerosol was transported about 11 feet to a minifold of l823 reclassifiers (No. 5 Alemi te nozzles, Alemite Division, Stewart-Warner Corporation, Chicago, Illinois). The condensed mist dripping off the reclassifiers was collected in a shallow beaker and weighed. Uncondensed mist (stray mist) was collected by vacuum through a filter paper which was weighed before and after testing. The amount of stray mist (the difference in weight of the filter paper before and after testing) times divided by the sum of the amount of stray mist and condensed mist is the percent stray mist. The test method is very sensitive in that it is capable of measuring percent stray mist to hundredths of a percent. Droplet-size distributions in regular and stray mists were quantitatively determined by means of an Anderson Air Sampler which is a commercial apparatus (2,000, Inc., Salt Lake City, Utah) developed to measure size distributions of aerosol particles. Samples for droplet-size distributions were taken only 2 feet from the mist generator in order not to omit those drops which condense in the connecting pipe to reclassification nozzles.

EVALUATION Table I shows the effect of polymer-type on the mist suppressant ability of the polymer or copolymer. Polymethacrylate of very high molecular weight present at 0.2 percent by weight in a neutral mineral oil gives 1.05 percent stray mist. However, a much lower molecular weight polypropylene is much more effective than the ester in stray mist suppression. Finally, the lower molecular weight C C copolymer is substantially more effective than the polypropylene at the same weight per- TABLE 1 Polymer-Type Effects Stray MW Conc. Mist 1. Polyalkylmethacrylate 500,000 0.20 1.05 2. C C; Copolymer 300,000 0.20 0.25 3. Polypropylene 50,000 0.20 0.42

Molecular weight Concentration in weight percent in a 500 SSU at 100F neutral petroleum oil Percent stray mist measured as described elsewhere Table 11 shows that while, in general, the higher the molecular weight, the more effective the polymers are as stray mist suppressants at the same concentration, e.g., see,polyisobutylene, this is not necessarily true of the preferred C C copolymers. These copolymers reach their most effective molecular weight for mist suppressant purposes at about 20,000 or greater and are not appreciably improved thereafter by increments in molecular weight.

TABLE II Effect of Polymer Molecular Weight Stray MW Colic. Mist 1. Polyisobutylene 50,000 0.20 0.50 2. Polyisobutylcne 800,000 0.20 0.35 3. C:C= 9mm 100,000 0.20 0.15 4. C C Copolymer 300,000 0.20 0.25

Molecular Weight Concentration in weight percent in a 500 SSU at 100F neutral petroleum oil Percent stray mist measured as described elsewhere Table III shows the marked concentration effect of decreasing stray mist with increasing polymer concentration at constant polymer molecular weight and type. Again, however, the effect is not so marked for the preferred C C copolymers which are as effective (very effective) as mist suppressants at 0.2 weight percent as at appreciably higher weight percents.

TABLE 111 Effect of Polymer Concentration Stray MW Cone. Mist 1. C,C, Copolymer 300,000 0.01 1.70 2. C,C, Copolymer 300,000 0.02 0.70 3. C,C, Copolymer 300,000 0.20 0.25 4. C,C, Copolymer 300,000 0.50 0.20 5. Polyisobutylene 50,000 0.02 1.20 6. Polyisobutylene 50,000 0.05 0.90 7. Polyisobutylene 50,000 0.10 0.65 8. Polyisobutylene 50,000 0.20 0.50 9. Polyisobutylene 50,000 1.00 0.35 10. Polyalkylmethacrylate 30,000 0.06 3.00

TABLE ill-Continued Effect of Polymer Concentration Molecular weight Concentration in weight percent in a 500 SSU at F neutral petroleum oil Percent stray mist measured as described elsewhere It has been shown that polymers added to a mist oil shift its droplet size distribution towards larger droplets (FIG. 2). This effect increases with increasing polymer concentration and increasing size of the polymer molecules. Polymer size can, however, be varied in at least two ways: first, by varying the molecular weight, and, second, by varying the unit or monomer weight. Thus two polymers of the same molecular weight, one consisting of polyester units each with a C side chain, and the other of propylene units, will have greatly differing molecular size. The polypropylene molecule will be much larger than the polyester since most of its atoms are in the backbone and contribute to its length while most of the atoms of the polyester are in the side groups and contribute more to its bulkiness. It is found that polymers with a thin backbone and the smallest side chains consistent with oil solubility, e.g. C C are much more effective in shifting the size distribution in oil mists to larger droplets, than polyalkyl methacrylates and long-chain polyolefins with bulky backbones and long side chains.

Using the slender polymers of the present invention, the effect of concentration is found to be large up to about 0.1 weight percent and then relatively and increasingly small towards higher concentrations. However, with bulky polymers, the concentration dependence is not so large at first, but it extends to higher concentrations before leveling off.

Proper choice of chemical structure, molecular weight and concentration permits an optimal suppression of the smallest droplets and relatively little diminution of the larger ones. This translates directly into suppression of stray mist without detriment to desirable mist. Direct comparisons with polyesters show that while it is possible to reduce stray mist to levels of about 1 percent if concentrations of the polyester additives reach about 1 percent, much lower concentrations of the preferred C C copolymers, i.e., 0.1 percent, reduce stray mist to about 0.2 percent, and consequently, are much more effective stray mist suppressants.

We claim:

1. A process of mist lubrication producing a low percentage of stray mist which comprises converting into a mist a composition comprising a major amount of an oil of lubricating viscosity and as a stray mist suppressant from 0.001 to 2 weight percent of an oil-soluble copolymer of ethylene with a C to C monoolefin, the copolymer having viscosity average molecular weight of at least 5,000; pneumatically transporting said mist to an area to be lubricated; and reclassifying said mist in said area.

2. The process of claim 1 wherein the copolymer is a copolymer of ethylene and a C -C monomeric mono-olefin, said copolymer consisting of 40-80 mol percent of ethylene.

which comprises adding to said lubricating oil from 0.001 to 0.1 weight percent of an ethylene-propylene copolymer having a viscosity average molecular weight between about 5,000 and 300,000.

6. The method defined in claim 5 wherein a sufficient amount of said copolymer is added to reduce the level of stray mist below about 0.5 percent. 

2. The process of claim 1 wherein the copolymer is a copolymer of ethylene and a C3-C6 monomeric mono-olefin, said copolymer consisting of 40- 80 mol percent of ethylene.
 3. The process of claim 1 wherein the copolymer is a copolymer of ethylene and propylene, said copolymer consisting of 40-80 mol percent of ethylene.
 4. The process of claim 3 in which the percentage of stray mist is less than 0.5 percent.
 5. In a method for lubricating a system wherein a lubricating oil is converted into a mist lubricant and the mist lubricant contacted with parts desired to be lubricated, the improvement for suppressing stray mist which comprises adding to said lubricating oil from 0.001 to 0.1 weight percent of an ethylene-propylene copolymer having a viscosity average molecular weight between about 5,000 and 300,000.
 6. The method defined in claim 5 wherein a sufficient amount of said copolymer is added to reduce the level of stray mist below about 0.5 percent. 